Process for forming hard anodic oxide film on aluminum base alloy

ABSTRACT

A HARD ANODIC OXIDE FILM IS FORMED UNIFORMLY ON AN ALUMINUM BASE ALLOY CONTAINING 1% BY WEIGHT OR MORE OF COPPER BY SUBJECTING THE ALUMINUM BASE ALLOY OT ANODIC OXIDATION USING AN ELECTROLYTE COMPRISING AN AQUEOUS SOLUTION CONTAINING 5 TO 40% BY WEIGHT OF AN AROMATIC SULFONIC ACID SUCH AS SULFOSALICYLIC ACID, NAPTHALENEDISULFONIC ACID, NAPHTHALENE-TRISULFONIC ACID, PHENOL SULFONIC ACID AND SULFOPHTHALIC ACID, AND 15 TO 40% BY WEIGHT OF SULFURIC ACID.

United States Patent M 3,804,731 PROCESS FOR FORMING HARD ANODIC OXIDEFIIM 0N ALUMINUM BASE ALLOY Kiyomi Yanagida and Tadashi Tsukiyasu,Nagoya, and Seitaro Iwata, Ibaragi, Japan, assignors to SumitomoChemical Company, Limited, Osaka, Japan No Drawing. Filed Sept. 1, 1972,Ser. No. 285,595

Claims priority, application Japan, Sept. 7, 1971, 46/69,528 Int. Cl.C231! 9/02 US. Cl. 204-58 6 Claims ABSTRACT OF THE DISCLOSURE A hardanodic oxide film is formed uniformly on an aluminum base alloycontaining 1% by weight or more of copper by subjecting the aluminumbase alloy to anodic oxidation using an electrolyte comprising anaqueous solution containing to 40% by weight of an aromatic sulfonicacid such as sulfosalicylic acid, napthalenedisulfonic acid,naphthalene-trisulfonic acid, phenol sulfonic acid and sulfophthalicacid, and 15 to 40% by weight of sulfuric acid.

This invention relates to a process for forming on an aluminum basealloy an anodic oxide film which is hard and excellent in corrosionresistance and rigidity.

Heretofore, there have been proposed many processes for forming anodicoxide films on aluminum and aluminum base alloys, in general. Forexample, processes using an aqueous sulfuric acid solutions or oxalicacid solutions have been well known. For formation of hard anodic oxidefilms, there have been well known a Kalcolor process using a mixedaqueous electrolyte of sulfosalicylic acid and sulfuric acid (U.S. Pat.3,031,387), a Duranodic 300 process using a mixed aqueous electrolyte ofsulfophthalic acid and sulfuric acid (British Pat. 962,048), a Veroxalprocess using a mixed aqueous electrolyte of sulfosalicylic acid, maleicacid and sulfuric acid (British Pat. 973,391), a Sumitone process usinga mixed aqueous electrolyte of p-phenolsulfonic acid and sulfuric acid(Japanese patent publication No. 29,954/64) and a process using a mixedaqueous electrolyte of naphthalenedisulfonic acid and sulfuric acid(U.S. Pat. No. 3,486,991). These processes are characterized, as isclear from the patent specifications, in that aluminum as an anode ischarged in an electrolyte comprising a mixed aqueous solution of asuitable aromatic sulfonic acid and a low concentration sulfuric acid, acathode is also charged in the electrolyte and then an electric currentis flowed to the electrolyte to form an anodic oxide film on the surfaceof aluminum and aluminum base alloys. However, the said processes havenot been satisfactory as processes for forming anodic oxide films oncopper-containing aluminum base alloys. That is, when copper-containingaluminum alloys, particularly those containing more than 1% by weight ofcopper, are treated according to the abovementioued processes, elutionof copper takes place during the anodic oxidation treatment to giveanodic oxide films which are imperfect and soft to be put into practicaluse. For example, Al-Si-Cu alloys have been widely used for variousaluminum alloy castings, and have been frequently used die castings orsand castings for cylinders, engine parts, bearings and the likeproducts which are required to be high in abrasion resistance.Nevertheless, anodic oxide films formed by subjecting the said aluminumalloy products to anodic oxidation treatment are not uniform, thin infilm thickness and low in hardness.

The present inventors made extensive studies or processses for forminganodic oxide films on aluminum base alloys. As the result, the inventorshave found that a uniform and hard anodic oxide film is formed on acopper- 3,804,731 Patented Apr. 16, 1974 containing aluminum base alloyby subjecting the alloy to anodic oxidation treatment in an aqueouselectrolyte containing an aromatic sulfonic acid and a highconcentration sulfuric acid which solution has not been known hitherto.Particularly, it has been clarified that a hard anodic oxide film havingan excellent quality is formed on an Al-Si-Cu alloy containing 6% byweight or more of silicon and 1% by weight or more of copper.

It is therefore the object of the present invention is to provide aprocess for forming a hard anodic oxide film on an aluminum base alloy,characterized by subjecting an aluminum alloy containing 1% by weight ormore of copper to anodic oxidation treatment using as an electrolyte anaqueous solution containing 5 to 40% by weight of an aromatic sulfonicacid and 15 to 40% by weight of sulfuric acid.

In the present invention, the aromatic sulfonic acid is an essentialcomponent for forming an anodic oxide film on an aluminum base alloy. Ifthe concentration of the aromatic sulfonic acid is less than 5% byweight, no effect of increasing the hardness of the resulting film canbe observed. Further, the aromatic sulfonic acid is difficultlydissolved to a concentration of more than 40% by weight in an aqueoussulfuric acid solution of such a concentration as adopted in the presentinvention. If the concentration of sulfuric acid is less than 15% byweight, not only no uniform anodic oxide film can be formed on acopper-containing aluminum alloy or a high silicon content aluminumalloy, but also the cell voltage becomes higher with formation andgrowth of the film to easily cause burning. If the concentration ofsulfuric acid is more than 40% by weight, the organic acid is lowered insolubility to decrease the hardness-improving effect of the aromaticsulfonic acid. A part of the sulfuric acid may be replaced by awater-soluble sulfate or bisulfate such as sulfate or bisulfate of analkali metal. The use of an electrolyte comprising an aqueous solutioncontaining 10 to 15% by weight of aromatic sulfonic acid and 15 to 25%by Weight of sulfuric acid gives particularly excellent effects. Thearomatic sulfonic acid used in the present invention is sulfosalicylicacid, naphthalenedisulfonic acid, napthalene-trisulfonic acid,phenolsulfonic acid, sulfophthalic acid or the like aromatic sulfonicacid having at least one sulfone group. The electrolyte used in thepresent invention may be incorporated with a small amount of acarboxylic acid such as oxalic or maleic acid. 1

Aluminum-copper alloys containing 1% by weight or more of copper whichare to be treated according to the present process are not particularlylimited in form but may be products of any forms such as castings,sheets, extrusions and the like. According to the present process, hardand uniform anodic oxide films can be formed on aluminum-silicon-copperalloys containing 6% by weight or more of silicon and 1% by weight ormore of copper on which such films have been diflicultly formedhitherto.

In the present process, the hardness of the resulting film tends tolower with increasing anodic oxidation temperature. Accordingly, theelectrolyte temperature is desirably below 20 C., more advantageouslywithin the range from 0 to 15 C. The current density is in the rangefrom 1 to 4 amp./dm. and the treatment period varies depending on thethickness of the resulting film.

Generally, anodic oxide films formed according to the present processare subjected to conventional sealing treatment. In case aluminum alloyproducts having anodic oxide films formed according to the presentprocess are to be used for pistons, cylinders, bearings, etc., however,it is sometimes preferable that the anodic oxide films are not subjectedto sealing treatment, since the films, which have not been subjected tosealing treatment, are high in oil retainability.

The present invention is illustrated in detail below with thickness andhardness from those formed according to reference to examples, but it isneedless to say that the the known processes, and that the presentprocess can invention is not limited to the examples. form hard anduniform anodic oxide films on copper- Aluminum alloy materials used inthe examples and containing aluminum base alloys on which complete thickin comparative examples are shown in Table 1. 5 films have beendifiicultly formed hitherto.

TABLE 1.CHEMICAL COMPOSI'IXggQ ND SHAPE OF TEST ALUMINUM Alloycomposition (weight percent) Zn Mg Mn Cr Fe Ni Ti B Al Shape 0.25 0.290.06 0.22 0.005 Balance. Casting. do Sheet.

Casting. Do.

EXAMPLE 1 EXAMPLE 8 The aluminum alloy 1 in Table 1 was degreased,etched and desmutted according to ordinary procedures, and thensubjected to anodic oxidation treatment at a current den- 20 o 2 i of 22 for 60 minutes at a temperature of xldation treatment at a currentdenslty of 2.5 A./dm.

for 60 minutes in an electrolytic bath at 5 to 7 C. 10 C. in an aqueouselectrolyte containing by weight of naphthalene-disulfonic acid, byweight of Contammg by welgtft sulfosahicychc acld and sulfuric acid and1% by weight of oxalic acid. In this 20% by welght of su'lfunc acldcase,the case, the cell voltage was gradually increased from 20 v. Voltagewas gradually Increased from 15 50 The The aluminum alloy 4 in Table lwas degreased according to an ordinary procedure, and then subjected toanodic to 25 The fil fo m d by Said anodic oxidation treat. film formedby said anodic oxidation treatment was subment was subjected to sealingby means of boiling water. J' ed t0 Seali g treatment by use of boilingwater. The The film had a thickness of about 60,5 and a hardness of filmhad a thickness of about 60,71 and a hardness of about about Hvz370 interms of Vickers hardness. HvzSOO in terms of Vickers hardness.

glgg f fg EXAMPLES 9-15 AND COMPARATIVE EXAMPLES 3-5 Example 1 wasrepeated, except that the electrolyte composition was varied. Theresults obtained were as set Example 8 Was repeated, except that theelfictrolyte forth in Table 2. composition, the treatment temperatureand the treatment TABLE 2 Current Electrolysis Thick- Hardness Kinddensity ness of of oxide of Electrolyte composition (amp/ Temp. Timeoxide film alloy (wt. percent) dmfl) C.) (min.) film (p) (Hv) Examplenumber:

2 1 {gapgthalene-trisulfonic acid 13 2'5 10 60 60 400 1 HZSO 2.5 10 e050 320 1 {H230 2. 5 1o 60 40 300 1 2 5 10 5o 40 350 H250 2.5 10 5o 20150 {H280 2.5 10 25 150 From Table 2, it is understood that the filmsformed time were varied. The results obtained were as set forthaccording to the present process are greatly difierent in in Table 3.

TABLE 3 Current Electrolysis Thiek- Hardness Kind density ness of ofoxide 0! (amp./ Temp. Time oxide film alloy Electrolyte composition (wt.percent) dmfl) 0.) (111111.) film (p) (Hv) Example number:

Naphthalene-disuli'onic acid 15 9 igrso u nn a3} a one sn 0 c ac 1o 4 52.5 5-7 40 40 450 4 {H280 2. 5 5-7 40 40 470 4 {H250 2. 5 10-12 55 420{H2804 20 2. 5 5-7 60 60 510 C00 1% {H2804 2. 5 5-7 00 45 480 {H250 202. 5 10-12 50 440 15 2. 5 10-12 60 0-50 410 4 llgzsggfiin ainifnnauniaapene su o eac 4 {H280 2.5 5-7 50 e 380 The aluminum alloys 2, 3 and 5shown in Table 1 were subjected to anodic oxidation treatment accordingto the present invention. The results obtained were as set forth inTable 4.

2 A process according to claim 1, wherein the electrolyte contains 10 to25% by weight of the aromatic sulfonic acid and to by weight of sulfuricacid.

3. A process according to claim 1, wherein the aluminum alloy contains6% by weight or more of silicon and 1% by weight or more of copper.

4. A process according to claim 1, wherein the anodic oxidationtreatment is effected at a temperature of from 0 to 15 C.

5. A process according to claim 1, wherein the anodic oxidationtreatment is efiected at a current density in the range from 1 to 4amp./dm.

TABLE4 Current Electroysis Thick- Hardness Kind density ness of of oxideWhat is claimed is:

1. A process for forming a hard anodic oxide film on an aluminum alloy,characterized by subjecting an aluminum base alloy containing 1% byweight or more of copper to anodic oxidation treatment at a temperaturebelow 20 C. using an aqueous electrolyte containing 5 to 40% by weightof an aromatic sulfonic acid selected from the group consisting ofsulfosalicyclic acid, naphthalene-disulfonic acid,naphthalene-trisulfonic acid,

6. A process according to claim 1, wherein the electrolytic bath isfurther incorporated with oxalic or maleic acid.

References Cited phenol-sulfonic acid and sulfophthalic acid and 15 toGERALD KAPLAN Primary Examiner by weight of sulfuric acid.

R. L. ANDREWS, Assistant Examiner

